1. Field of the Invention
This invention relates to an impregnation process using a bio-templating method for nano-catalyst incorporation into the electrode(s) of a solid-state electrochemical cell. This technology more specifically provides an infiltration protocol based on the incorporation of organic bio-templating agent which assistances in the homogenous deposition of inorganic oxide nano-catalyst within a porous electrode(s) of a solid-state electrochemical cell.
2. Brief Description of the Background Art
In the fuel cell art, solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) have been improved by modification of their electrodes (anode/cathode) by impregnation (infiltration) of the pores of the electrode materials with nanoparticles. Such a modification allows for added catalytic function and enhancement of electronic/ionic conduction pathways which result in improved cell performance and reliability, and provide for an increase in the amount of triple phase boundaries (TPB) for both electrodes allowing for a decrease in polarization resistance and a subsequent increase in the kinetic performance of the fuel cell.
In the literature, three prominent impregnation methods that have been assessed so far (Sholklapper et al., Fuel Cells, 2008). Those are nano-particle suspension/dispersion infiltration, molten salt infiltration and metal salt precipitation.
Nano-particle suspension is the least preferred method and only applicable for highly porous and opened structured electrodes. In this method, nano-catalyst particles are dispersed in a suspension and impregnated into the electrode. One challenge is that in liquid medium, the surface of nano-particles are charged and the surface chemical properties are influenced by many factors, such as the nature of the ceramic particles, particle size distribution, surface impurities, solvent and pH value etc. (Fengqiu et al., Ceram. Int., 2000). This factor brings nano-particle agglomeration problems which decreases the effectiveness of the protocol.
On the other hand, the molten salt method permits the impregnation at a very high concentration level (measured in grams per impregnation step). Typically for this process, the inorganic salt is melted in an inert atmosphere (˜100-150° C.), and the processes often requires vacuum or pressure assistance to remove residual gas within the porosity and to force the highly viscous melt into the microstructure. Due to the viscosity issues and the localized non-homogenous deposition, pore clogging and gas diffusion problems were identified. It is noted that for both methods discussed above, the electrode itself acts as a filter in this method that inhibits homogenous deposition down the deep regions of electrodes which often leads to pore clogging and gas starvation issues (Sholklapper et al., Fuel Cells, 2008). Moreover, controlling the deposition kinetics and decreasing the number of infiltration cycles/steps, and lowering the time and labor are the basic challenges.
The last method, metal salt precipitation, is the most performed and preferred way of infiltration/impregnation of nano-catalyst into electrode microstructures. In this method, the catalyst salt, mostly nitrate and chloride salts, is dissolved into a liquid medium. Typically, an aqueous 0.05 M to 5 M salt solution is then introduced to the top of the electrode surface. In conventional “dripping method”, this step is performed in multiple repetitions to achieve a specific mass loading of the nano-catalyst within the microstructure. Moreover, due to the fast drying conditions of small amount of precursor, the liquid pulls the free cation to the near surface site, and hence, most of the nano-catalyst portion is and localized near the surface region after firing.